Fix restriction for PartTestTemplate

- limit_choices_to should be "testable", not "trackable"
- ref: inventree#7888

src/backend/InvenTree/part/models.py

@@ -3605,7 +3605,7 @@ def validate_unique(self, exclude=None):
         Part,
         on_delete=models.CASCADE,
         related_name='test_templates',
-        limit_choices_to={'trackable': True},
+        limit_choices_to={'testable': True},
         verbose_name=_('Part'),
     )